A diffractive light modulator is used to modulate an incident beam of light. One such diffractive light modulator is the Grating Light Valve™ (GLV™) device of Silicon Light Machines, Inc. of Sunnyvale, Calif. Device drivers provide control signals to the diffractive light modulator which instruct the diffractive light modulator to appropriately modulate the light beam incident thereto. The diffractive light modulator is connected to the device drivers via wire bonds, where each wire bond is connected to one bond pad on the diffractive light modulator and a corresponding bond pad on the device drivers. A conventional diffractive light modulator assembly, as illustrated in FIG. 1, consists of a diffractive light modulator chip 10 and four separate driver die 12, 14, 16 and 18. Each driver die 12, 14, 16 and 18 is coupled to the diffractive light modulator chip 10 by a plurality of wire bonds 11. The diffractive light modulator is built on its own process on silicon. The diffractive light modulator includes moveable elements and each element is connected to a corresponding bond pad. The diffractive light modulator is an essentially passive device where voltage is applied to make the elements move. In contrast, the device drivers are active. Each of the device drivers includes a plurality of transistors with appropriate layers of interconnects. The device drivers receive digital data and convert it to an analog response in the form of analog voltage. The analog voltage is then applied to the appropriate bond pad, which is then received by the corresponding element on the diffractive light modulator. In this manner, the device drivers provide control signals to the diffractive light modulator, thereby dictating the movement of the various elements.
In the field of light modulating devices, each element on the diffractive light modulator corresponds to a pixel within the light modulating device. For example, in the case of 1088 pixels, 1088 wire bonds are needed as input to the diffractive light modulator from the device drivers. 1088 wire bonds requires 272 bond pads on the output side of each of the four device drivers. However, it is much easier to perform high density wiring using standard semiconductor processing steps then it is to do wire bonding. Since only 60-70 wire bonds are necessary on the input side of each of the device drivers, it would be advantageous to internally wire the connections between the device drivers and the diffractive light modulator on the same chip. In this manner, it would only be necessary to have the 60-70 wire bonds as inputs to this integrated chip, thereby eliminating the additional 1088 wire bonds of the conventional diffractive light modulator assembly. By reducing the number of wire bonds, the manufacturing process is made easier. Further, fewer wire bonds reduces the packaging cost of each device. Still further, by eliminating the wire bonds between the device drivers and the diffractive light modulator, types of device driver designs whose functionality and/or speed was previously limited by the parasitic capacitance or inductance of the wire bonds can now be used.
There is also a reliability problem associated with such a high number of wire bonds. Since there is a finite failure rate associated with each wire bond, the more wire bonds there are, the greater the chance that one of the wire bonds will fail. Reducing the number of wire bonds would necessarily reduce the number of failing wire bonds, and increase the reliability of the device. Physically, each bond pad leaves a footprint. As such, the size of the diffractive light modulator assembly is determined in great part by the total number of bond pads. If the number of bond pads is reduced, the size of the diffractive light modulator assembly can also be reduced. As the device is bond pad limited, there is a significant amount of wasted real estate. Since this wasted real estate exists on silicon which can be used to manufacture the device drivers, the device drivers could be manufactured on the real estate currently being used by the bond pads.
Electro-static discharge (ESD) protection is usually incorporated into active devices ranging from diodes to transistors and integrated circuits. It is a matter of layout and design to add ESD protection structures to the pad during transistor fabrication on the integrated circuits. This protection prevents the circuitry from being damaged by ESD. However, since there is no active device on the diffractive light modulator chip, there is no ESD protection. As a result, a significant amount of yield is lost during manufacturing of the diffractive light modulators due to ESD induced “snap-downs.” In a snap-down, the pad on the diffractive light modulator acts as an antenna and sees an ESD event. The ESD event is regarded as a voltage by the element on the diffractive light modulator and the element is snapped down thereby destroying itself. It would be advantageous to incorporate ESD protection into the normal manufacturing process of the diffractive light modulator.
Considering the above shortcomings, it is clear that if the device drivers are integrated onto the same silicon monolithically with the diffractive light modulator, then this would produce a significant advantage.
Unfortunately, the manufacturing processes of the device drivers and the diffractive light modulator are not the same. Further, by integrating the device drivers and the diffractive light modulator onto the same silicon substrate, significant manufacturing problems are introduced.
Conventional transistor manufacturing processes are described below in relation to FIGS. 2 and 3. FIG. 2 illustrates an exemplary transistor used in the device drivers of the diffractive light modulator assembly. The transistor illustrated in FIG. 2 is early in the manufacturing process and is often referred to as the front-end of the transistor. In a first step, silicon dioxide films 22 are grown on a silicon substrate 20. Next, a gate 24 and source-drain 26 are added by manufacturing processes that are well known in the art of semiconductor fabrication. A next step, as illustrated in FIG. 3, is deposition of an oxide layer 30 over the front-end of the transistor. The oxide layer 30 is then planarized, typically by a chemical-mechanical polishing technique. Contact holes are then etched in the oxide layer 30 to access the gate 24 and the silicon substrate 20, for example. Metalization is performed for the wiring of the device drivers. Metalization is typically performed by sputtering a metal layer over the oxide layer 30, patterning and etching the metal layer to form contacts 32 and 34. Another oxide layer 36 is then deposited and planarized. Contact holes are etched in the oxide layer 36 to access the contacts 32 and 34. Metalization is then performed to form the contacts 38 and 40. Additional layers of oxide and metalization are added as determined by the design considerations of the device. Typically, there are 3-5 layers of metal which form the interconnects of the device drivers.
Conventional diffractive light modulator manufacturing processes are described below in relation to FIGS. 4-7. The first step, as illustrated in FIG. 4, is the deposition of an insulating layer 51 followed by the deposition of a sacrificial layer 52 and a silicon nitride film 54 on a silicon substrate 56.
In a second step, as illustrated in FIG. 5, the silicon nitride film 54 is lithographically patterned into a grid of grating elements in the form of elongated elements 58. After this lithographic patterning process, a peripheral silicon nitride frame 60 remains around the entire perimeter of the upper surface of the silicon substrate 56. After the patterning process of the second step, the sacrificial layer 52 is etched, resulting in the configuration illustrated in FIG. 6. It can be seen that each element 58 now forms a free standing silicon nitride bridge. The elements of a diffractive light modulator, such as elements 58, are also referred to as “ribbons”. As can further be seen from FIG. 6, the sacrificial layer 52 is not entirely etched away below the frame 60 and so the frame 60 is supported above the silicon substrate 56 by this remaining portion of the sacrificial layer 52.
The last fabrication step, as illustrated in FIG. 7, is sputtering of an aluminum film 62 to enhance the reflectance of both the elements 58 and the substrate 56 and to provide a first electrode for applying a voltage between the elements and the substrate. A second electrode is formed by sputtering an aluminum film 64 onto the base of the silicon substrate 56. Alternatively, the second electrode can be introduced earlier in the process by sputtering an aluminum film onto the upper portion of the silicon substrate 56 prior to deposition of the insulating layer 51.
In FIGS. 8 and 9, an alternative embodiment of a conventional diffractive light modulator is illustrated. In this embodiment the diffractive light modulator consists of a plurality of equally spaced, equally sized, fixed elements 72 and a plurality of equally spaced, equally sized, movable elements 74 in which the movable elements 74 lie in the spaces between the fixed elements 72. Each fixed element 72 is supported on and held in position by a body of supporting material 76 which runs the entire length of the fixed element 72. The bodies of material 76 are formed during a lithographic etching process in which the material between the bodies 76 is removed.
The problem is how to manufacture the diffractive light modulator on the same chip as the transistors that comprise the device drivers. Combining a diffractive light modulator and its associated device drivers onto a monolithically integrated device using conventional manufacturing process steps would be advantageous.